The present invention relates to ferromagnetic thin-film structures and, more particularly, to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics.
Many kinds of electronic systems make use of magnetic devices. Digital memories are used extensively in digital systems of many kinds including computers and computer systems components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital bits as alternative states of magnetization in magnetic materials in each memory cell, particularly in cells using thin-film magnetic materials, resulting in memories which use less electrical power and do not lose information upon removals of such electrical power.
Magnetometers and other magnetic sensing devices are also used extensively in many kinds of systems including magnetic disk memories and magnetic tape storage systems of various kinds. Such devices provide output signals representing the magnetic fields sensed thereby in a variety of situations.
Such memory cells and sensors can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic states, or magnetic conditions, therein. Such devices may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the device and the operating circuitry therefor.
Ferromagnetic thin-film memory cells, for instance, can be made very small and packed very closely together to achieve a significant density of information storage, particularly when so provided on the surface of a monolithic integrated circuit. In this situation, the magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetization fields which can cause instabilities in the magnetization state desired in such a cell.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin-film memory cells can be ameliorated to a considerable extent by providing a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory thin-film is provided. Such an arrangement provides significant "flux closure," i.e. a more closely confined magnetic flux path, to thereby confine the magnetic field arising in the cell to affecting primarily just that cell. This is considerably enhanced by choosing the separating material in the ferromagnetic thin-film memory cells to each be sufficiently thin. Similar "sandwich" structures are also used in magnetic sensing structures.
In the recent past, reducing the thicknesses of the ferromagnetic thin-films and the intermediate layers in extended "sandwich" structures having additional alternating ones of such films and layers, i.e. superlattices, have been shown to lead to a "giant magnetoresistive effect" being present. This effect yields a magnetoresistive response which can be in the range of up to an order of magnitude greater than that due to the well-known anisotropic magnetoresistive response.
In the ordinary anisotropic magnetoresistive response, varying differences between the direction of the magnetization vector in the ferromagnetic film and the direction of the sensing current passed through the film lead to varying differences in the effective electrical resistance in the direction of the current. The maximum electrical resistance occurs when the magnetization vector in the film and the current direction are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. The total electrical resistance in such a magnetoresistive ferromagnetic film can be shown to be given by a constant value, representing the minimum resistance, plus an additional value depending on the angle between the current direction in the film and the magnetization vector therein. This additional resistance follows a square of the cosine of that angle.
As a result, operating external magnetic fields can be used to vary the angle of the magnetization vector in such a film portion with respect to the easy axis of that film portion which comes about because of an anisotropy therein typically resulting from depositing the film in the presence of a fabrication external magnetic field oriented in the plane of the film along the direction desired for the easy axis in the resulting film. During subsequent operation of the device with the resulting film, such operating external magnetic fields can vary the angle to such an extent as to cause switching of the film magnetization vector between two stable states which occur as magnetizations oriented in opposite directions along that easy axis. The state of the magnetization vector in such a film portion can be measured, or sensed, by the change in resistance encountered by current directed through this film portion. This arrangement has provided the basis for a ferromagnetic, magnetoresistive anisotropic thin-film to serve as part of a memory cell.
In contrast to this arrangement, the resistance in the plane of a ferromagnetic thin-film is isotropic with respect to the giant magnetoresistive effect rather than depending on the direction of a sensing current therethrough as for the anisotropic magnetoresistive effect. The giant magnetoresistive effect has a magnetization dependent component of resistance that varies as the cosine of the angle between magnetizations in the two ferromagnetic thin-films on either side of an intermediate layer. In the giant magnetoresistive effect, the electrical resistance through the "sandwich" or superlattice is lower if the magnetizations in the two separated ferromagnetic thin-films are parallel than it is if these magnetizations are antiparallel, i.e. directed in opposing directions. Further, the anisotropic magnetoresistive effect in very thin-films is considerably reduced from the bulk values therefor in thicker films due to surface scattering, whereas very thin-films are a fundamental requirement to obtain a significant giant magnetoresistive effect.
In addition, the giant magnetoresistive effect can be increased by adding further alternate intermediate and ferromagnetic thin-film layers to extend a "sandwich" or superlattice structure. The giant magnetoresistive effect is sometimes called the "spin valve effect" in view of the explanation that a larger fraction of conduction electrons are allowed to move more freely from one ferromagnetic thin-film layer to another if the magnetizations in these layers are parallel than if they are antiparallel with the result that the magnetization states of the layers act as sort of a valve.
These results come about because of magnetic exchange coupling between the ferromagnetic thin-films separated by the intermediate layers, these intermediate layers typically formed from a nonferromagnetic transition metal. The effect of the exchange coupling between the ferromagnetic thin-film layers is determined to a substantial degree by the thickness of such an intermediate layer therebetween. The effect of the coupling between the separated ferromagnetic thin-film layers has been found to oscillate as a function of this separation thickness between these layers in being ferromagnetic coupling (such that the magnetizations of the separated layers are parallel to one anther) and antiferromagnetic coupling (such that the magnetizations of the separated layers are opposed to one another, or antiparallel to one another). Thus, for some separation thicknesses, the layer coupling will be of zero value between extremes of such oscillations.
Exhibiting the giant magnetoresistive effect in a superlattice structure, or in an abbreviated superlattice structure formed by a three layer "sandwich" structure, requires that there be arrangements in connection therewith that permit the establishment alternatively of both parallel and antiparallel orientations of the magnetizations in the alternate ferromagnetic thin-film layers therein. One such arrangement is to have the separated ferromagnetic thin-films in the multilayer structure be antiferromagnetically coupled but to a sufficiently small degree so that the coupling field can be overcome by an external magnetic field.
Another arrangement is to form the ferromagnetic thin-film layers with alternating high and low coercivity materials so that the magnetization of the low coercivity material layers can be reversed without reversing the magnetizations of the others. A further alternative arrangement is to provide "soft" ferromagnetic thin-films and exchange couple every other one of them with an adjacent magnetically hard layer (forming a ferromagnetic thin-film double layer) so that the ferromagnetic double layer will be relatively unaffected by externally applied magnetic fields even though the magnetizations of the other ferromagnetic thin-film layers will be subject to being controlled by such an external field.
One further alternative arrangement, related to the first, is to provide such a multilayer structure that is, however, etched into strips such that demagnetizing effects and currents in such a strip can be used to orient the magnetizations antiparallel, and so that externally applied magnetic fields can orient the magnetizations parallel. Thus, parallel and antiparallel magnetizations can be established in the ferromagnetic thin-films of the structure as desired in a particular use. Such a structure must be fabricated so that any ferromagnetic or antiferromagnetic coupling between separated ferromagnetic films is not too strong so as to prevent such establishments of film magnetizations using practical interconnection arrangements.
A broader understanding of the giant magnetoresistance effect, i.e. the spin valve effect, can be obtained by considering a generalized multilayer structure shown in FIG. 1 and ignoring, for simplicity though this is not necessary, the ordinary anisotropic magnetoresistive effect. The structure is typically provided on a semiconductor chip, 10, having suitable operating circuitry therein. An electrical insulating layer, 11, supports N identical ferromagnetic thin-film conductive layers, each separated from an adjacent one by one of N-1 identical nonmagnetic, conductive intermediate layers to form a superlattice structure. A highly resistive outer passivation layer, 13, covers this structure, and suitable electrical interconnections are made to the conductive layers but not shown. The conductance of this superlattice structure will be the sum of the conductances of the individual layers which are effectively electrically in parallel with one another, but the giant magnetoresistive effect introduces magnetization dependence in the ferromagnetic thin-films. In the following, a possible model is developed to an extent as a basis for gaining a better understanding of the electrical and magnetic behavior of this structure, but this model is simplified by approximations and not all would agree with every aspect of the approach chosen.
The conductance of very thin-films is highly dependent on surface scattering if the mean-free path of conduction electrons in the bulk material of the films is equal to or longer than the thickness of the films. The ratio of the film conductivity to the conductivity of the film material in bulk can be expressed as a function of the ratio of the film thickness to the mean-free path of electrons in bulk material identical to the film material by the well known Fuchs-Sondheim conduction model assuming inelastic scattering at the film surfaces, or by other associated models taking further conditions into account such as grain boundary scattering and other surface scatterings.
The magnetization dependence in the ferromagnetic thin-films leading to the giant magnetoresistive effect appears dependent on the ratio of spin up to spin down electrons in the 3D shell of the transition elements used in the ferromagnetic thin-films, i.e. the spin polarization P of the conduction electrons. The fraction f of 3D electrons which are spin up have typical values of 0.75 for iron, 0.64 for cobalt and 0.56 for nickel. Conduction electrons in metals are normally S shell electrons which theoretically would be equally divided between spin up and spin down electrons. However, because of band splitting, the conduction electrons in the magnetic layers are assumed to have a fraction of spin up electrons like that of the electrons in the 3D shell. The spin polarization is then determined from P=2f-1. Such electrons are assumed in encounters with atomically perfect boundaries between the magnetic layers, including in this boundary the thin nonmagnetic, conductive intermediate layer therebetween, to be either scattered inelastically or pass freely into the next magnetic layer.
In view of the observed spin polarization, the simplifying assumption is made that the probability of a spin up electron not being scattered in entering a material having a majority of spin up electrons is approximately equal to the fraction of the electrons in the conduction band which are spin up, and that the probability of a spin down electron going into the same material not being scattered is equal to the fraction of the electrons in the conduction band which are spin down. Changing the magnetization directions between parallel and antiparallel in adjacent ferromagnetic thin-films changes the conduction band electrons in the films from having matching spin up and spin down fractions in each to having opposite spin up and spin down fractions in each. Thus, a larger fraction of the electrons in the superlattice structure will be scattered when the magnetizations in the ferromagnetic thin-films are antiparallel as compared to when they are parallel, since more than half of the electrons in the conduction band are spin up in view of the spin up fraction values given above. If the ferromagnetic thin-films are separated by a conductor layer which preserves the spin of the conduction electrons in passing therethrough, some conduction electrons can pass from one layer to the other without collisions and so can travel through effectively a thicker layer than those which are scattered to thereby be confined within a single layer. As a result, the scattered electrons can follow paths having a significantly lower conductivity and so, if the ferromagnetic films are oppositely magnetized, there will be a greater effective resistance in the structure. This view of the conduction electron transport between ferromagnetic thin-film layers can be adjusted for imperfections at the boundaries between adjacent ferromagnetic thin-films for conduction band electrons, which would not be scattered because of the spin thereof, may instead be scattered by physical imperfections at the boundary.
Based on the foregoing, the effective conductivities for parallel and antiparallel magnetization states in the superlattice structure can be determined, and subtracted from one another to provide the ratio of change in effective conductivities of the ferromagnetic thin-films, due to a corresponding change between parallel and antiparallel magnetizations in those films, to the average conductivity in those films. The result of this determination must have added to it the conductivities of the nonmagnetic, conductive intermediate layers on the basis of those layers having equal populations of spin up and spin down conduction band electrons, and a conductivity which does not change with magnetization directions. In such a setting, the ratio of the difference in sheet conductances of the superlattice structure when the ferromagnetic thin-films change magnetization from parallel to antiparallel, .DELTA..gamma..sub..perp..fwdarw..rect-ver-solid., to the average of these sheet conductances, .gamma..sub..perp..fwdarw..rect-ver-solid., can be obtained as ##EQU1## ignoring the ordinary anisotropic magnetoresistance in obtaining this giant magnetoresistive response as indicated above. Here q represents physical boundary imperfections, and is the probability that a conduction electron which would not be scattered because of its spin is also not scattered by physical imperfections or collisions in the nonmagnetic, conductive intermediate layers.
The symbol .gamma..sub.m1 is the sheet conductance in a single ferromagnetic thin-film, the sheet conductance per unit square of a thin-film being the conductivity thereof multiplied by the thickness thereof. Thus, N.gamma..sub.m1 is the sheet conductance of N parallel ferromagnetic thin-films. The symbol .gamma..sub.mN is the sheet conductance of a layer of ferromagnetic thin-film N times the thickness of a single ferromagnetic thin-film, and .gamma..sub.c1 is the sheet conductance of a nonmagnetic, conductive intermediate layer.
The number N of ferromagnetic thin-films affects the differences in sheet conductances because of the difference in conductivity between a ferromagnetic thin-film which is N layers thick compared to N ferromagnetic thin-films electrically connected in parallel. The polarization factor P is, as indicated above, expected to be important in the giant magnetoresistive response in representing the fraction of spin up conduction band electrons, and this expectation is borne out by the square of that factor appearing in the numerator of the equation above.
The quality of the interface between the ferromagnetic thin-films and the nonmagnetic, conductive intermediate layers is important as represented in the last equation by the symbol q. The largest giant magnetoresistive effect values have been obtained in material systems where both the lattice constant of the crystal and the form in the crystal class of each interface material have been well matched. For example, chromium matches the body-centered cubic structure of iron better than all other natural body-centered cubic nonmagnetic metals. Copper is similarly the best match for face-centered cubic cobalt and for face-centered permalloy mixtures which are quite similar to nickel. Significant mismatches will likely give a very low value for q.
Also, scattering in the nonmagnetic, conductive intermediate layers is likely if the thickness of those layers is smaller than the mean-free path of conduction electrons in the bulk film material. 10 Hence, the symbol q will be reduced in value for thicker intermediate films.
The film thickness also has a significant effect on the ratio of .gamma..sub.mN /.gamma..sub.ml with this ratio increasing as the films get thinner, as shown by the Fuchs-Sondheim conduction model. The greatest conductivity difference between parallel and antiparallel magnetizations in the ferromagnetic thin-films can be seen, from the last expression above, to occur in the very thinnest of magnetic layers were it not for the scattering and shunting effects of the nonmagnetic, conductive intermediate layers. However, once the conductance of the magnetic layers, decreasing in being made thinner, gets to be on the order of the conductance of the nonmagnetic, conductive layers, the expression above shows that further decreases in thickness will reduce the giant magnetoresistive effect. Thus, for a fixed set of parameters for the nonmagnetic, conductive intermediate layer, the giant magnetoresistive effect will have a peak in value at some ferromagnetic thin-film thickness.
This assumes that the coupling between the structure ferromagnetic thin-films is also arranged to result in an operable device since it determines the range of magnetization angles which can be achieved in a device for given values of applied magnetic fields, so sets limits on the magnetoresistive response. If positive, or ferromagnetic, coupling is present and too great, the film magnetizations will not be sufficiently close to being antiparallel, and perhaps cannot be made so by passing a sensing current through the structure, so that the maximum resistance expected for the configuration cannot be obtained. On the other hand, if negative, or antiferromagnetic, coupling is present and too great, the film magnetizations will not be sufficiently close to being parallel, and perhaps cannot be made so by applying an external magnetic field to the structure, so that the minimum resistance expected for the configuration cannot be obtained.
Further, there is a limit on the thinness of the nonmagnetic, conductive intermediate layer because of "pin holes" occurring therethrough which result in that layer covering less than 100% of the surfaces of the ferromagnetic thin-films on either side thereof. These "pin holes" in the nonmagnetic, conductive intermediate layers are thought to lead to a current density dependence in the giant magnetoresistive effect which is not reflected in the last expression above. Such pin holes in this intermediate layer appear to result in ferromagnetic coupling between the ferromagnetic thin-films on either side of that layer in the vicinity of such holes thereby creating ferromagnetically coupled magnetic domains in these ferromagnetic thin-films which are otherwise antiferromagnetically coupled (assuming no external magnetic fields being applied).
As a result, there appears to be an incomplete saturation of magnetizations across the superlattice along the easy axes so that higher currents through the superlattice structure generate a "scissoring" magnetic field (a field forcing magnetizations in films adjacent an intermediate layer in opposite directions) which counteracts the effects of the pin holes by forcing the magnetizations in the pin hole domains to more closely align with the magnetizations in the rest of the ferromagnetic thin-film in which they occur. Sufficiently high currents can leave a single domain in each such ferromagnetic thin-film.
Although the effect of a very low pin hole density can be perhaps corrected by providing a sensing current of a sufficient current density through the superlattice structure, a relatively small increase in pin hole density will quickly lead to all of the ferromagnetic thin-films being ferromagnetically coupled so that the magnetizations therein are in, or near to being in, a common direction. Such a result will make the superlattice structure inoperable as a device, and so there is a desire to provide thin nonmagnetic, conductive intermediate layers with reduced pin hole densities.